The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
One key characteristic of small (<5 nm) silicon nanoparticles is that these particles are photoluminescent in visible light when stimulated by a source of lower wavelength light. This is thought to be caused by a quantum confinement effect that occurs when the diameter of the nanoparticle is smaller than the exciton radius, which results in bandgap bending (i.e., increasing of the gap). Although silicon is an indirect bandgap semiconductor in bulk, silicon nanoparticles having a diameter below five nanometers emulate a direct bandgap material. Since direct bandgap materials can be used in optoelectronic applications, silicon nanoparticles have the potential to become a key material for use in future optoelectronic applications. However, the preparation of nanoparticles with surface properties that allow for prolonged stabilization, i.e., minimize agglomeration, and that also exhibit maximum emission intensity and luminescent quantum efficiency has been and still is a persistent challenge.
Academic and industrial laboratories have conducted and continue to conduct research directed towards the development of manufacturing methods and reactors that can be used to produce nanoparticles and modify the properties exhibited by the nanoparticles formed therefrom. Several of these methods include the use of microreactor plasma, aerosol thermal decomposition of silane, ultrasonication of etched silicon, laser ablation of silicon, and plasma discharges. Plasma discharges provide an opportunity to produce nanoparticles either at high temperatures using atmospheric plasma or at approximately room temperature using low pressure plasma. Luminescent silicon nanoparticles have been produced using ultrahigh vacuum and very high frequency (e.g., radiofrequency) capacitively coupled plasmas. The very high frequency allows for the coupling between the radio frequency (rf) power and the discharge to produce a high ion density and ion energy plasma. However, such a capacitively coupled system requires a relatively high input power (˜200 W) in order to provide plasma that exhibits even modest power (˜5 W) because a large amount of the input radiofrequency power is reflected back to the power supply.